Process for the desiliconization of manganese alloys

ABSTRACT

The invention concerns a process of desiliconization of manganese alloys in the liquid state. 
     By injecting carbon dioxide into the liquid alloy, which injection can be effected by an additional neutral gas, or oxidizing agent, the silicon is oxidized to SiO 2 . The addition of lime or dolomite favors the slagging of the silicon. By this process the silicon content can be as low as 0.1%. 
     The process is particularly applicable for obtaining a ferromanganese with low carbon and low silicon content.

The present invention concerns a process for the desiliconization bymeans of carbon dioxide of manganese alloys and in particularferromanganese alloys, in the liquid state.

Manganese alloys which are intended for siderurgical uses are producedby two broad types of process:

When carbon-saturated alloys are to be produced, manganese ore istreated in an electric furnace or in a blast furnace, with one or morecarbon-bearing reducing agents.

When the alloys to be produced are not carbon-saturated, a manganese andsilicon alloy is reacted on a manganese ore in the presence of lime.These reactions may be carried out in an electric furnace similar tothose used in steel marking or in a ladle in which the manganese-siliconalloy is reacted with a molten mixture of lime and manganese ore.

In these two production processes, the resulting product is aferromanganese which has a greater or lesser silicon content and whosesilicon content is in equilibrium with the residual content of manganeseoxide slag. In accordance with the mass action law, applied to thereaction:

Si+2MnO⃡SiO₂ +2Mn

the losses of manganese in the slag increase in proportion as thesilicon content of the final metal falls.

In order to comply with the requirements made by those in thesiderurgical industry, attempts have been made to reduce the amount ofsilicon in manganese-base addition alloys. Many studies have beencarried out and published, all of which aimed to reduce the losses inrespect of manganese in the slag, for a given silicon content in thecommercial alloy. The most effective process consisted of increasing thebasicity number of the slag by increasing its proportion of lime. Thismethod suffers from disadvantages since, on the one hand, it contributesto increasing the volume of the slag and on the other hand it increasesits melting temperature, that is to say, it results in the operatingtemperature of the metallurgical apparatus being higher and the lossesof manganese due to volatilization being higher.

Another solution to the problem of low silicon content comprisesproducing alloys which are not carbon-saturated by injecting oxygen intoa carbon saturated base alloy which therefore has a low silicon content.This process of decarbonization with pure oxygen, as described inparticular in French Pat. Nos. 2,167,520 and 2,317,369 in the name ofGesellschaft fur Elektrometallurgie NBH, suffers from the disadvantageof causing severe losses of manganese by volatilization and does notmake it possible to achieve very low final carbon proportions, undereconomically satisfactory conditions.

The present invention concerns a novel process for producing manganesealloys with a very low silicon content, which is applied to allmanganese alloys whether carbon-saturated or not.

This process comprises treatment in the liquid state of the manganesealloy which is to be desiliconized by carbon dioxide which reacts on thesilicon which is to be removed with sufficiently moderate exothermicityfor the degree of volatilization of the manganese to remain very low.Besides the substantial advantage which this process provides byreducing the manganese losses due to volatilization, this process alsomakes it possible to limit the losses of manganese in thedesiliconization scoria as the carbon monoxide produced by the reaction:Si+2CO₂ →SiO₂ +2CO provides for intense mixing as between the metal andthe scoria which accordingly are in almost perfect chemical equilibrium.

According to the stoichiometry, 44.8 liters of CO₂ are required tooxidize 28 grams of silicon, that is to say, 1.6 m³ of CO₂ per kg ofsilicon. In practice, we use from 1 to 3 times and preferably from 1 to2 times the stoichiometric amount of CO₂, and 0.5 times and preferably0.7 times the stoichiometric amount of CO₂ when a gas capable ofoxidizing silicon is used in combination with the CO₂ to make up thebalance.

The invention can be carried into effect in any chamber whatever, whichwe shall refer to hereinafter generally as a "reactor." The walls of thereactor are formed by a refractory cladding, preferably of the magnesiumtype. The shape of the reactor is not of determining importance, but itis preferable for the reactor shape to have symmetry of revolution.During the desiliconization treatment, the axis of symmetry may bevertical or slightly inclined, and the reactor may be stationary or mayrotate about its axis. In order to provide optimum content between thecarbon dioxide and the manganese alloy to be desiliconized, it ispreferable for the height of liquid alloy in the reactor to be greaterthan the diameter of the top surface. For the same reason, it ispreferable for the carbon dioxide to be introduced at the bottom of thereactor by means of a pipe positioned in the side wall, adjacent thebottom, or disposed in the actual bottom of the reactor, or by any otherknown equivalent means.

In order to promote the desiliconization reaction: Si+2CO₂ →SiO₂ +2CO itis possible to add lime (CaO) which is intended to scorify the silica,in proportions such that the final CaO/SiO₂ ratio is from 0.8 to 2.5.The lime may be added either in the powder state in suspension in thecarbon dioxide, or in the form of pieces, at the surface of the alloy tobe treated. The addition of lime may be totally or partially replaced bythe addition of calcium carbonate, the thermal decomposition of which,at the temperature of the reaction, provides both the carbon dioxide andthe calcium oxide required.

It is possible for the addition of lime to be accompanied by additionsof manganese oxide or manganese ore, which are intended to limit thedegree of scorification of the manganese contained in the alloy beingtreated, in a proportion of from 3 to 15% by weight of the treatedalloy. When the reaction temperature would rise to such an extent thatthere would be a fear of losses of manganese due to volatilization, itis also possible to add amounts of ferromanganese in powder or pieceform, in order to reduce the temperature, in a proportion of between 0.5to 10% by weight of the alloy to be treated.

Finally, the addition of lime may be partly or totally replaced in anaddition of crude dolomite (CaCO₃, MgCO₃) or calcined dolomite (CaO,MgO) which makes it possible somewhat to reduce the degree of wear ofthe refractory materials of the reactor, when they are of magnesiumtype.

Although desiliconization can be achieved by injecting pure carbondioxide, it has been found that it was possible for the action of thisgas to be strengthened, modulated or completed by associating therewithmake-up gases such as pure oxygen, air, nitrogen, argon or steam. Bysuitably selecting the make-up gas, it is possible to control thetemperature, eliminate parasitic gases which are contained in the alloyor achieve secondary chemical or physical-chemical effects. When atleast one oxidizing gas other than carbon dioxide is used as the make-upgas, it is possible to reduce the proportion of CO₂ which is introduced,below the stoichiometric amount, for example down to 0.5 and preferably0.7 times stoichiometry. The remainder of the desiliconization action isthen produced by the oxidizing make-up gas or gases referred to above.The make-up gases may be used at the same time as the carbon dioxide orsequentially. In the former case, they can be introduced in mixture withthe carbon dioxide or by means of a double pipe comprising for exampletwo coaxial members. Thus, when treating manganese alloys with a lowcarbon content, it is preferable to dilute the carbon dioxide with aninert gas such as argon in order to prevent recarbonization of thealloy.

The following example makes it possible more clearly to demonstrate anembodiment of the invention:

EXAMPLE 1

A tonne of ferromanganese having the composition set out below is to bedesiliconized:

Si: 1.0%

C: 0.9%

Mn: 82.7%

Fe: balance

The treatment is carried out in a cylindrical reactor comprisingmagnesia bricks joined with a carbon-bearing paste, being 0.75 m indiameter and 1.25 m in height. The thickness of the liquidferromanganese layer in the reactor is about 0.35 m. Injection of thecarbon dioxide is effected by means of a blast pipe which is 14.5 mm indiameter and which opens horizontally into the reactor at about 5 cmabove the bottom thereof.

The treatment comprises injecting 20 normal cubic meters of carbondioxide, over a period of 15 minutes. During the first 12 minutes, theCO₂ is associated with oxygen, in a proportion of 1 m³ of oxygen for 3m³ of CO₂.

During the last 3 minutes, the CO₂ is injected alone, so as to controlthe temperature of the bath and to limit volatilization of themanganese.

In addition, during the operation, 30 kg of CaO and 60 kg of manganeseore are added.

After treatment, 975 kg of alloy is obtained, containing:

Si: 0.12%

C: 0.95%

Mn: 81.80%

Fe: balance

After cleaning, the scoria is recovered so that it can be used in theproduction of silico-manganese. The desiliconized ferromanganese is castin an ingot mold after optionally having been subjected to deoxidizationby means of aluminum.

I claim:
 1. A process for desiliconization of manganese base alloyscontaining silicon characterized by injection into said alloy which isliquid and disposed in a reactor, an amount of carbon dioxide which isfrom about 1 to about 3 times the stoichiometric amount which permitsoxidation of silicon in accordance with the reaction: Si+2CO₂ →SiO₂+2CO, thereby removing silicon and affecting an alloy having a very lowsilicon content.
 2. A process for the desiliconization of manganese basealloys in accordance with claim 1 characterized in that the amount ofCO₂ injected is from about 0.5 to about 0.7 times the stoichiometricamount.
 3. A process for the desiliconization of manganese base alloysin accordance with claim 1 characterized in that a basic substance isintroduced into the reactor during the injection of CO₂, to scorify thesilica formed by oxidation of the silicon.
 4. A process for thedesiliconization of manganese base alloys in accordance with claim 3characterized in that the basic substance is calcium oxide in an amountsuch that the final scoria has a CaO/SiO₂ ratio of from about 0.8 toabout 2.5.
 5. A process for the desiliconization of manganese basealloys in accordance with claim 3 characterized in that the basicsubstance is at least partly crude or calcined dolomite.
 6. A processfor the desiliconization of manganese base alloys in accordance withclaim 3 characterized in that the basic substance is calcium carbonatewhose thermal decomposition in the reactor provides at least a part ofthe lime and the CO₂ required for desiliconization.
 7. A process for thedesiliconization of manganese base alloys in accordance with claim 3characterized in that the basic substance introduced in powder form isentrained in the flow of carbon dioxide.
 8. A process for thedesiliconization of manganese base alloys in accordance with claim 1,claim 2, claim 3, claim 4, claim 5, claim 6 or claim 7, characterized byintroducing into the reactor an oxygen-bearing manganese compound in aproportion of from about 3% to about 15% by weight of the alloy to bedesiliconized.
 9. A process for the desiliconization of manganese basealloys in accordance with claim 8, characterized in that ferromanganesein powder or piece form is introduced into the reactor, in a proportionof from about 0.5% to about 10% by weight of the alloy to bedesiliconized.
 10. A process for the desiliconization of manganese basealloys in accordance with claim 9, characterized in that the action ofthe carbon dioxide is completed by at least one make-up gas selectedfrom air, oxygen, nitrogen, argon and steam.
 11. A process for thedesiliconization of manganese base alloys according to claim 10characterized in that the make-up gas is introduced simultaneously withthe injection of CO₂.
 12. A process for the desiliconization ofmanganese base alloys in accordance with claim 10 characterized in thatthe make-up gas is introduced after the injection of CO₂.
 13. A processfor the desiliconization of manganese base alloys in accordance withclaim 1 wherein said manganese base alloy is a ferromanganese alloy.